Learning characters is not difficult in the sense that it requires you to be intelligent. It just takes a long time. What makes a language difficult is the complexity of grammar, which is why I believe Japanese is considerably more difficult than Chinese, despite Japanese having an "alphabet" while Chinese does not.
Russian is a trap, and very difficult. It has a complex case system with some crazy noun declension rules, complex verb conjugation (tense, aspect, and gender), and context-dependent word order. It shares a lot of similarities with Japanese for these reasons.
Vietnamese would be another trap because it's harder than other Asian languages despite using the Roman alphabet. Six tones. Ouch.